Apparently it has been upgraded to an f / 1.6 sensor for better low light performance. (indianexpress.com)
The camera gets an upgrade as well improving low light performance, so that's a small yet nice addition for games that make heavy use of the feature. (animalcrossingworld.com)
The Honor 8 already had great low light performance in my testing thanks to the monochrome sensor, so it's fair to assume the Mate 9 will be better. (thenextweb.com)